The present invention relates generally to application of laser absorption spectrometry for isotope ratio analysis of gases and more specifically to laser absorption spectroscopy based systems and methods.
Optical absorption spectroscopy involves passing radiation through a sample, e.g., an analyte and measuring absorption property of the sample as a function of the radiation wavelength. For example, trace gas detection can be spectroscopically performed by taking measurements to detect the presence or absence of spectral absorption lines corresponding to the gas species of interest. Trace gas detection can be spectroscopically performed by taking measurements to quantify spectral absorption lines corresponding to the gas species of interest and to compute concentrations of analytes, gas pressure, and gas temperature. Spectroscopic analysis of isotopologues can also be performed. However, because the line intensities of rotational-vibrational lines are sensitive to the gas temperature, and the line shapes of those lines are also sensitive to the gas pressure and to gas composition, measurements of the isotopic ratio with high accuracy require highly accurate measurements of the analyzed gas temperature and pressure. Moreover, because the natural abundance for isotopes can be very different, the line intensities of absorption gas lines of different isotopologues can also be very different.
The current procedures of measuring the ratio of isotopologue concentrations in the gas phase by the optical spectroscopy method consist of measuring of optical spectra of a test sample, deriving the concentrations of different isotopogues in the tested gas based on the measured spectra, fitting them using spectral databases and line-shape models; measuring of optical spectra of one or more calibrated test samples, deriving the concentrations of different isotopogues in the calibrated samples based on the measured spectra, fitting them using spectral databases and line-shape models, and calculating the ratio of different isotopogues in the test sampe based on these measurements. A common procedure to measure the ratio of concentrations of different isotopologues in a gas mixture comprises periodical calibrations of an optical absorption based gas analyzers by using gas mixtures having known composition of the isotopologues. This procedure is rather expensive, time consuming, and it requires precisely calibrated gas mixtures.
Accordingly, it is desirable to provide improved spectroscopy methods for measuring concentrations of different isotopologues in their gas phase.